The following paragraphs are not an admission that any of the information below is common general knowledge or citable as prior art.
Selenium is an essential trace element, but becomes toxic at very low concentrations. Selenium accumulates in the bodies of plants and fish that live in selenium-contaminated water and in the bodies of wildlife and people that eat those plants and fish. In people, elevated selenium concentrations may cause neurological damage and hair and nail loss.
Selenium has been treated in biological reactors, for example as described in U.S. Pat. No. 6,183,644 and International Publication Number WO 2007/012181, and as used in ABMet™ reactors sold by the GE Water and Process Technologies. In such reactors, dissolved selenium is removed from contaminated water by treating the water in a reactor containing selected endemic and other selenium reducing organisms. Microbes may be isolated from the specific water or imported from other selenium contaminated water. The microbes are then screened for ability to reduce selenium under the site specific environmental conditions. The selected microbes are optimized for selenium reduction, then established in a high density biofilm within a reactor. The selenium contaminated water is passed through the reactor with optimized nutrient mix added as needed. The elemental selenium is precipitated and removed from the water. The entirety of U.S. Pat. No. 6,183,644 is incorporated herein by this reference to it.
The high density biofilm may be supported on a media bed, as described in US Publication No. 2013/0270181. Activated carbon may be employed as the medium and provides a large surface area available for microbial growth. The activated carbon may be in the form of granular activated carbon (GAC) or pelletized activated carbon. Other media may be used, for example polymeric fibers, crushed stone, pumice, sand, plastic media or gravel. As selenium, and possibly other solids, accumulate in the media bed, the pressure drop across the media bed will increase. As a selected time interval or pressure drop set point, the media bed is backwashed to remove the grown biomass and solids retained in the media bed. The upflow velocity during backwashing may be about 80 ft/hr. Backwashing may be required from between once every two weeks to only a few times each year, for example once a month. Backwashing may take, for example, 30 minutes. The upflow velocity applied during flushing may result in an upward expansion of the media bed by up to 30%. The backwash liquid and entrained solids are removed through troughs located above the expected media expansion area and connected to backwash effluent line. The entirety of US Publication No. 2013/0270181 is incorporated herein by this reference to it.